Designing your printed circuit boards, PCB comes with one crucial step: choosing the appropriate surface finish. You will get the help of protecting the copper circuitry from corrosion using the OSP PCB surface finish. Not only that, your components will have a solderability surface through the PCB surface finish. And many users face the challenge of picking a PCB coating material that they can easily and quickly solder the electronic components onto the circuit board. Users need to consider some of these factors:
- Cost
- Environmental impact
- Requirements for durability
- Expected production volume
- The components you use
Some of the types of PCB surface finishes during PCB manufacturing include:
- HASL
- ENIG
- OSP
- ENEPIG
- lSn
- iAg
Because of its environmental-friendly and low-cost qualities, OSP is getting more popularity than other finishes. This prevalence adds more compulsion for people to understand the meaning and importance of OSP better.
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What is the OSP PCB Surface Finish?
OSP includes the process of treating the surface of the printed circuit board (PCB) copper foil with regards to the RoHS instructions. It also uses a water-based organic compound that rusts in a typical setting. However, there’s a need to use the flux to remove the protective film with ease at the subsequent welding temperatures. The molten solder can contain the exposed clean copper surface immediately to form a solid spot quickly.
Uses and Applications Suited for OSP PCB Surface Finishes
The OSP surface finish has strengths and weaknesses that make it suitable for these use cases.
Low Volume Prototypes
Applying OSP surface finishes on circuit boards is relatively cheap compared to the alternatives and requires little equipment. So, it can save you some money if you’re only producing low-volume prototypes.
You can also use this finish for engineering testing before you start commercial production to evaluate the product.
An electrical engineer holding a circuit board prototype
Quick-Turn Fabrication
Applying this organic finish takes a short time, resulting in a quick turnaround time.
Double-Sided Boards
These boards require protective coats on two sides, and OSP is suitable because it is cheaper than other surface finishes and will make the PCBs more affordable.
Manual Soldering
The OSP finish maintains good wetting and solderability of the copper traces even after 6-12 months of storage.
Therefore, the OSP-coated surfaces are suitable for hand soldering even if you don’t want to mount the components immediately.
Furthermore, these finishes allow rework and repairability of solder joints because fresh solder will still wet the pads well.
A technician hand-soldering a circuit board
Automated Assembly
The good wetting property of this surface finish forms strong solder joints that can survive handling on automated assembly lines when using pick-and-place machines, conveyor belts, etc.
Short Shelf Life Designs
The 6 to 12-month shelf life of OSP surface finishes is relatively short compared to other metal finishes, which means you should use this option if your boards ship quickly.
Circuit boards stored on a tray for future use
Materials of OSP PCB Surface Finish
The primary materials of OSP are:
- Rosin
- Active resin
- Azole
The process uses oxidation to maintain the copper surface. It is possible to apply the water-based organic compound’s thin protective layer over the exposed copper. The use of a conveyorized technique is vital to achieving this process.
The azole family boasts the water-based chemical compound, like benzimidazoles, imidazoles, and benzotriazoles. They all use coordination formed between the copper atoms and them to absorb on the copper surface, which leads to the creation of a protective film. Since it doesn’t need sunlight exposure, it requires a moderate temperature of about 15 to 30 degrees and relative humidity of about 30 to 70 percent RH for the optimal storage conditions. The OSP surface finish requires some of these steps:
- Rinse
- Topography Enhancement
- Rinse
- Acid Rinse
- OSP application
- Deionization Rinse
- Dry
Manufacturing Process of OSP PCB Surface Finish
OSP’s manufacturing process includes necessary steps like clean, topography enhancement, acid rinse, OSP application, and dry. These steps will prepare the PCB board for a smooth OSP coating application.
Clean
The process of removing organic contaminants like oxidation film, fingerprints, oil, and so on, to get a clean PCB board.
Topography Enhancement
By performing micro-etching, it will improve the bonding forces between OSP film and exposed copper. Also, it minimizes the oxidation generated on copper.
Deionization Rinse
Ions will populate the OSP solution before the final OSP application. With this, it will be easy to eliminate during soldering.
Advantages of OSP Surface Finish
The fact that OSP surface finishes have lead-free applications and VOC-free material makes it people’s choice.
As a result, it is reliable against moisture from a long-term point of view. With three I.R. soldering cycles, anyone can maintain its solderability.
Generally, it has 2 to 5 rounds of reflow soldering before degradation. OSP might be the best choice for anyone who wants to protect their exposed copper on their PCB’s mid-production.
Similarly, since it doesn’t interact with gold surfaces, they can also use OSP in conjunction with other technologies. Some other advantages are:
Simple and Reworkable Manufacturing Process
The PCB fabricators can rework the OSP-coated circuit boards with ease. As a result, fresh coatings will be available for PCB assemblers when there is damage to its layer.
Excellent Wettability
When flux metal meets pads and vias, OSP-coated boards tend to function better.
Low Cost
When it comes to cost, people favor OSP among the rest of the surface finishes. Similarly, they get a lower cost of the circuit boards since it costs less.
SMT Assembly Responsive
It can handle the SMT components easily since it is lead-free.
Flat Surface
The coplanar surface it offers is well-suited for tight-pitch pads like QFP and BGA.
Environment Friendly
In the process of OSP generation, there is an application of a water-based compound, which falls into people’s expectations for a green world and causes no harm to the environment. Therefore, OSP caters to green regulations like RoHS when it comes to optimal selection for electronic products.
Solder Mask Ink Low Requirement
OSP needs low requirements when it comes to solder mask ink.
Durability
The long storage period is one of the main advantages of OSP.
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Disadvantages of OSP Surface Finish
The main disadvantage of OSP surface finish is its susceptibility to mechanical damage. Thus, it requires careful handling. Since it is colorless and transparent, it is hard to inspect. Also, it is virtually impossible to have a visual inspection of the OSP finish. Users need to handle the boards using gloves since sweat and water dew could quickly decompose it.
The variety of actual performance and recipes are some of its other shortcomings. Users must carefully transport and manipulate the protective film as it is prone to bruises or scratches since it is fragile. In other words, an unwelded plate of OSP film can crack or change color, thus impacting reliability and weldability. Other disadvantages of OSP include:
Difficulty in Thickness Measurement
It is hard to measure OSP thickness because it is a transparent film. It is not easy to find the impact of not protecting the copper surface since the film thickness is too thin. Thus, welding can be impossible since the film thickness is too narrow.
Not Suitable for Plated Through-Hole
As it has less than six months because of its short shelf-life, it is not suitable for PTH (Plated Through Holes).
Highly Sensitive
OSP is highly sensitive, and a little contact with water or moisture can damage it.
Short Shelf-Life
The OSP’s self-life is not more than six months after its completion. As for others, users may not use them for more than three months.
Users can return some boards that exceed the board’s shelf-life, depending on their quality and capabilities.
The factory will re-add a new OSP after washing off the old OSP on the PCB surface. However, it will require more corrosive chemicals to wash off the old OSP.
As a result, it will more or less damage the copper surface. Therefore, if they cannot process the solder pad because it is too small, owners must confirm with the board manufacturer if they can resurface it.
Cause ICT Issues
For users to assemble the process, it requires significant changes. It is not ICT friendly, and there can be potential damage to the PCB with any aggressive ICT fixture probes.
It also compromises repeatability with a lot of ICT limits, and they need manual handling precautions.
OSP is also not popular with most assemblers because of the significant changes in the assembly process.
Users must also handle it with care because fingerprints can quickly erode the OSP finish, exposing the copper to oxidation.
Exposed Cu on Final Assembly
The recommendation is to operate in an open nitrogen environment for secondary reflow to get a good welding effect.
How OSP Protects Copper From Oxidation
OSP is basically an organic acid that reacts with the outer copper layer to form a protective layer that is only a few atoms thick. This layer prevents humidity and moisture from getting to the copper below to corrode or oxidize it.
However, this layer only lasts for about 6-12 months, after which it will expose the bare copper metal to oxidation. So you must ship the PCB quickly or assemble it before this lifespan lapses.
OSP Impact on Soldering and Solderability
One of OSP surface finish's most important properties is that it solders very well. This phenomenon occurs because the thin protective layer vaporizes easily when exposed to high temperatures; it does not form a thermal barrier.
During reflow or manual soldering, the high heat removes this layer to expose the copper and get considerably good wetting.
An infrared reflow soldering oven
However, the layer doesn’t get eliminated. Each soldering cycle eats up part of this solder, which means you will expose the bare copper pad after multiple soldering cycles.
OSP can only withstand 4-6 reflow cycles before being depleted, so a lot of rework will damage this finish.
Possible OSP PCB Finish Problems After Soldering
Sometimes, the color of OSP surface finishes can change after soldering, which is usually caused by the thickness of the protective layer, soldering time, contaminants, and micro-etching quantity.
You can tell the severity of this issue by observing the board’s appearance.
If the pad’s color tarnishes to have a darker but uniform color, you don’t have to worry.
The flux you applied before soldering is capable of eliminating this minimal oxidation, resulting in strong solder joints.
Soldering tools, including rosin flux in a syringe dispenser
But if the color changes from brown to dark brown and you can spot some oxidation spots, the OSP layer’s reliability is most likely compromised, which means lower solderability performance.
You can take certain measures to ensure the solderability performance and surface finish appearance is normal.
The first is to control the protective layer’s thickness within a specific range. You should also control the micro-etching to be within a specific range and eliminate all contaminants that can mess up the solderability process.
Storage Requirement of PCBs Coated with OSP
OSP needs adequate care during transportation and operation because OSP technology’s generated preservatives can cut as thin.
With the long exposure of PCBs with OSP as a surface finish to humidity and high temperature, there is a possible generation of oxidation on PCBs’ surface.
As a result, it can result in low solderability. Thus, some of the principles of soldering technique to use include:
- Users must use a humidity display card and desiccant to utilize a vacuum package. They can also stop friction from destroying the PCB surface by placing release paper between PCBs.
- Users must not directly expose these PCBs to the sunlight. For an optimal storage environment, the requirements are storage time of fewer than 12 months, the temperature of about 15 to 300C, and relative humidity of about 30 to 70 percent RH.
What Should You Pay Attention to When Assembling OSP PCBs?
Solderability
This factor is critical because it determines how well the molten solder will attach and bond to the solder pad. While considering solderability, remember to check and analyze the possible OSP problems after soldering.
Duration of Applying the Surface Finish
The time taken to apply the surface finish matters, as well. OSP is an organic acid that corrodes the copper to form the protective coat, so you should only run the process for a short time to avoid corroding too much of the copper.
In comparison, surface finishes like HASL take longer to ensure all the copper surfaces get an even solder coat when dipped in the molten metal bath.
Reliability
It is important to measure the reliability of the OSP surface finish to ensure it preserves the solder pad and copper for the longest time possible. You can measure the reliability factor by checking or testing how vulnerable the coating surface is to corrosion.
OSP vs. ENIG vs. HASL PCB Surface Finish
OSP isn’t as popular as ENIG and HASL because it has a shorter shelf life of 6-12 months. But it is cheap to apply, provides good solderability, is RoHS compliant, and forms a flat surface for soldering. However, it provides poor aluminum wire bonding.
ENIG is the most expensive of the three because it involves immersing the pads in gold. But it is highly popular because it has a shelf life of over one year, is RoHS compliant, provides good solderability, forms a flat soldering surface, and provides good aluminum wire bonding.
A PCB with an ENIG gold surface finish
HASL sits somewhere between OSP and ENIG cost wise and is non-RoHS compliant if you use Leaded solder in the molten bath.
Additionally, it does not form a flat soldering surface and does not bond to aluminum wire bonds.
But it is popular because it has the best solderability and a long shelf life of over one year.
Conclusion
The focus is on environmental protection with the rapid development of society.
With this, the PCB industry has reduced pollution. The leading causes of corruption are some distinct surface finishes, such as HASL.
Before 2005, the focus was on lead since lead was the source of almost all the PCBs.
However, it was a harmful substance that impacts the air and water.
Since then, there has been pressure on manufacturers from some institutions and government to remove the HASL and replace it with HASL lead-free.
Unfortunately, there will be an increase in the cost of PCB manufacturing with the new process.
Therefore, the OSP application will continue to be the mainstream to protect the environment and cut costs.
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